Medical sleuths puzzling over three related bird flu cases in Thailand last fall now strongly believe that two women who cared for a sick child both caught the virus from the girl.

This is not the first evidence of human-to-human transmission of the virus, which has killed more than three dozen people in Thailand and Vietnam since the outbreak last year. In 1997, scientists believe bird flu also spread between people in rare cases in Hong Kong.

Also, last week, Vietnamese officials were investigating another suspected person-to-person bird flu case involving two brothers in Hanoi.

People normally catch bird flu from infected birds, usually chickens and ducks. International health experts are not worried about limited person-to-person transmission. Their biggest fear is a mutation of the virus into a form that passes easily between people, which could lead to a deadly flu pandemic. So far, there is no evidence the virus is changing into a more dangerous form.

In a study, an international team of scientists led by the Public Health Ministry in Thailand believed that an 11-year-old Thai girl became ill with the virus last fall and probably passed it on to her mother and aunt. The girl and the mother died 12 days apart. The aunt survived.

World health experts previously had theorized the girl passed the H5N1 virus on only to her mother and that it was a dead-end case. This is the first time that scientists have said the aunt also probably caught the virus from the child. But they have found no evidence of the virus mutating.

Details of the study will appear in Thursday's New England Journal of Medicine, which released the findings early to coincide with a conference at the University of Michigan on Monday.

The girl was hospitalized last September with a fever, cough and sore throat. She died the next day and was cremated without being tested for bird flu. Thai health officials believed she got the virus from free-ranging chickens in the house she and her aunt shared in a village in the northern province of Kamphaengphet.

The mother, who didn't live with them and had no exposure to poultry, visited her daughter's bedside where nurses reported seeing them hug and kiss. The mother, who got sick days after the girl died, died almost two weeks later. An autopsy showed the mother tested positive for bird flu.

Meanwhile, the aunt had gotten rid of the chickens days before the girl became ill. Throat swabs later confirmed the aunt also had bird flu. Health investigators believe she could not have caught it from the chickens and conclude she got it from the child.

Last year, bird flu struck 10 Asian countries, killing or forcing the slaughter of more than 120 million birds and causing more than 30 deaths in Thailand and Vietnam.

In the coming months, researchers at seven U.S. university sites will start the first ever human testing of experimental bird flu vaccines made by Chiron Corp. and Aventis Pasteur, the vaccine division of French-based Sanofi-Aventis.